1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and a high frequency (radio frequency) coil for irradiation of an electromagnetic wave or detection of a magnetic resonance signal.
2. Background Art
An MRI apparatus is a medical diagnostic imaging apparatus that causes magnetic resonance of atomic nuclei in an arbitrary cross section of an inspection target and acquires a tomographic image of the cross section, from the magnetic resonance signal produced in the magnetic resonance. If a living body placed in a static magnetic field is irradiated with a high frequency magnetic field from a high frequency coil (RF coil) while applying a gradient magnetic field to the living body, atomic nuclei in the living body, such as hydrogen nuclei, are excited. The excited hydrogen nuclei generate a circular polarized magnetic field as a magnetic resonance signal when the nuclei are restored to the equilibrium state. The signal is detected by the RF coil and processed to produce an image of the proton distribution in the living body. In order to acquire an image of higher precision and higher resolution, there is a demand for higher irradiation efficiency and higher reception sensitivity of the RF coil used for irradiation of the electromagnetic wave or detection of the magnetic resonance signal.
The quadrature detection (QD) method can improve the irradiation efficiency and reception sensitivity of the RF coil (see Patent Document 1 and Non-Patent Documents 1 and 2, for example). In the QD method, two RF coil disposed so that the axes thereof are perpendicular to each other are used to detect a magnetic resonant signal. The two coils disposed so that the axes thereof are perpendicular to each other are referred to as QD coil. For example, the QD coil may be composed of two loop coils partially overlapping each other (see Non-Patent Document 3, for example). Alternatively, the QD coil may be composed of a loop coil and a figure-of-eight coil (see Non-Patent Document 4, for example).
Since the magnetic resonance signal is a circular polarized magnetic field, when the QD coil detects the magnetic resonance signal, the two RF coils detect signals shifted in phase by 90 degrees. If the two detected signals are combined by shifting the phase of one of the signals by 90 degrees, the signal-to-noise ratio of the signal is ideally improved by a factor of √2 compared with the case where the signal is detected by only one RF coil. Furthermore, the power required for irradiation of the high frequency magnetic field is reduced to half, high frequency heat generation of the human body can be reduced.
Non-patent Document 5 discloses a coil that is composed of two loop coils disposed geometrically perpendicularly to each other and one coupling coil disposed to be inductively coupled to each of the two loop coils and is capable of quadrature phase detection by adjusting the degree of coupling between the loop coils and the coupling coil so that the phase difference between the signals detected by the two loop coils is 90 degrees.
[Patent Document 1] JP Patent No. 3095402
[Non-Patent Document 1] C. N. Chen et al., “Quadrature Detection Coils—A further √2 Improvement in Sensitivity”, Journal of Magnetic Resonance, Vol. 54, pp. 324-327 (1983)
[Non-Patent Document 2] G. H. Glover et al., “Comparison of Linear and Circular Polarization for Magnetic Resonance Imaging”, Journal of Magnetic Resonance, Vol. 64, pp. 255-270 (1985)
[Non-Patent Document 3] ZahiA. Fayad et al., “An improved Quadrature or Phased-Array Coil for MR Cardiac Imaging”, Magnetic Resonance in Medicine, Vol. 34, pp. 186-193 (1995)
[Non-Patent Document 4] K. Vij et al., “A Quadrature Neck Coil Array”, Proceedings of the International Society of Magnetic Resonance in Medicine, Vol. 1994, pp. 1108 (1994)
[Non-Patent Document 5] S. M. Wright, “Circular Polarization from Receiver Coils with a Single Coupling Loop: A simple Technique for Quadrature Detection”, Proceedings of Seventh Annual Meeting (Works in Progress) Society of Magnetic Resonance in Medicine, p. 137 (1988)